Ion sensitive solid state device and method

ABSTRACT

Ion-sensitive solid-state device having a ferroelectric body with a discontinuous thin film on the same and electrodes for placing a voltage across the body to cause polarization to occur so that ions adsorbed by the ferroelectric will change the resistivity of the discontinuous film.

United States Patent [1 1 Fehlner Feb. 19, 1974- [54] ION SENSITIVE SOLID STATE DEVICE 3,045,198 I 7/1962 Dolan et al. 338/35 X AND METHOD 3,251,654 5/1966 Palmer ...-340/237 R X 3,237,181 2/1966 Palmer 340/237 R Inventor: Francis P. Fehlner, Sunnyvale, Calif.

Assignees: Signetics Corporation, Sunnyvale,

Calif.; Corning Glass Works, Corning, N.Y.

Filed: July 16, 1971 Appl. No.: 163,234

US: Cl 338/34, 73/23, 117/212, 200/6103 Int. Cl H01c 13/00 Field of Search 338/34, 35; 73/73, 19, 23, 73/3365, 338; 340/237, 235; ZOO/61.03, 61.04, 61.06; 117/212; 324/33 Primary ExaminerC. L. Albritton Attorney, Agent, or FirmFlehr, Hohbach, Test, A1- britton & Herbert [57] ABSTRACT Ion-sensitive solid-state device having a ferroelectric body with a discontinuous thin film on the same and electrodes for placing a voltage across the body to cause polarization to occur so that ions adsorbed by the ferroelectric will change the resistivity of the discontinuous film.

8 Claims, 7 Drawing Figures PATENTEDFEBWIBH 3.793.605

VIII/IIIIIIIIIIIIIIIIII/l'.'IIIIIIII/I0 'IIII/III/Il Attorneys ION SENSITIVE SOLID STATE DEVICE AND METHOD BACKGROUND OF THE INVENTION Some efforts have been made for detecting constituents of certain gases and liquids. This has become particularly important in pollution control. There is a sig nificant need for simple and reliable devices which can be utilized for this purpose.

- SUMMARY OF THE INVENTION AND OBJECTS continuous thin film. Another electrode is provided on the other side of the body so that a voltage can be applied across the body to polarize the body toward or away from the discontinuous thin film so that the ferroelectric will adsorb ions of one polarity or the other to change the resistance of the discontinuous thin film.

In the method, the discontinuous thin film is evaporated ata relatively high vacuum at a temperature of approximately 200C. to cause agglomeration of the metal to form the discontinuous thin film'on the ferroelectric body.

In general, it is an object of the present invention to provide an ion-sensitive solid-state device and method which is relatively simple to fabricate and which is inexpensive.

Another object of the invention is to provide a device and method of the above character which are relatively simple.

Another object of the invention is to provide a device of the above character which can be utilized for analyzing-both gases and liquids.

Another object of the invention is to provide a device of the above character which can be utilized in relatively high temperature environments.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING completed device incorporating the present invention and also showing electrical circuitry connected to the same.

FIG. 5 isa plan view of the device shown in FIG. 4. FIG. 6 is a cross-sectional view of another embodiment of the invention showing an encapsulated version.

FIG. 7 is a cross-sectional view of another embodiment of the device showing the same mounted on a semiconductor device.

DESCRlPTlON OF THE PREFERRED EMBODlMENT The gas sensor of the present invention is constructed by first taking a body 11 which is of a bulk ceramic, glass ceramic or single crystal material that has planar upper and lower surfaces 12 and 13. The body 11 has a thickness which is chosen so that it will have sufficient structural strength and still operate with a relatively low voltage. In order to achieve polarization as hereinafter explained, it is necessary to provide an electric field in the range of 1X10 volts per centimeter which normally would require approximately volts across the body. Typical materials which could be utilized for the body 11 are the classical ferroelectrics such as barium titanate or barium strontium niobate and polarizable dielectrics such as lead magnesium niobate and lead nickel niobate. The basic desired qualities for such a material are that it has a relatively high resistivity and a high remnant polarizability. The resistivity should be above lXlO ohmcm. and the remnant polarizability should be greater than 1 microcoulomblcm There is no upper limit on the resistivity or the remnant polarizability. In fact, the higher each of these is, the better the device will work. Thus, to achieve excellent characteristics, the resistivity should be as high as 1X10 ohmcm. and the remnant polarizability should be as great as 50 microcoulombslcm With these characteristics in mind, the body 11 should be chosen so that it has a depth ranging from /i to 1 mil.

Electrodes may then be formed on the body 11. However, it should be understood that, if desired, the electrodes can be formed at a later stage in the process. Layers 14 and 16 of a suitable metal can be deposited on the surfaces 12 and 13. Metals which would be suitable for this purpose would be chromium or gold. By way of example, the metal being utilized can be evaporated in a vacuum chamber onto the surfaces 12 and 13 to form layers of a suitable thickness as, for example, 5,000 Angstroms. Thereafter, as shown in FIG. 3, by the use of suitable photolithographic techniques and masks, the upper spaced electrodes 17 and 18 can be formed on the top surface 12 and an electrode 19 can be formed on the bottom surface 13. Alternatively, if desired, the electrodes 17 and 18 and 19 can be formed on the bottom surface 13. Alternatively, if desired, the electrodes 17 and 18 and 19 can be formed by evaporating the desired metal through masks to form the desired pattern of electrodes. In addition, the electrodes can be formed by silk screening a paste or ink containing the desired metal onto the surfaces 12 and 13 and thereafter firing the same so that only the desired metal remains in the desired pattern on the surfaces 12 and 13.

After the electrodes 17, 18 and 19 have been formed, the body 11 can be mounted upon a suitable substrate 21 which can either be a metal or an insulator.

A discontinuous thin metal film 26 is then formed on the upper surface 12 between the electrodes 17 and 18. The discontinuous film can be deposited in a suitable manner. For example, the conducting material which is utilized can be evaporated in a vacuum chamber to form the desired discontinuous thin film. Evaporation is the preferable manner because the metal will deposit on nucleation centers and growth of the metal film will automatically give the desired size of islands and interisland spacing which will maximize the effect of ion adsorption on the ferroelectric which is to be utilized in the present device. The discontinuous thin film can best be made from noble metals so that permanent changes in resistivity will not be encountered due to oxidation of the islands. Oxidation of the islands would lead to an irreversible change in film resistance. If gold is selected, it should be evaporated at a high vacuum of 10 Torr or above at a rate of 5 to 20 Angstroms per minute. For gold, the islands will nucleate and grow to form a distribution of island sizes ranging from 100 to 5,000 Angstroms in diameter and inter-island spacings ranging from 20 to 100 Angstroms. There will be a distribution of islands and inter-island spacings of the various sizes with the peak number of islands and interisland spacings being at intermediate figures. In addition to gold, such noble metals as paladium and platinum can be used.

The desired resistance for the discontinuous thin film can be obtained quite precisely by. monitoring the resistance of the discontinuous thin film during the deposition and annealing process. This monitoring may induce some change in the film structure but it will not affect the electron motion through the film and substrate. The electron motion will still continue to be random.

lt'is desirable that when the discontinuous thin film is being deposited, it be deposited at an elevated temperature as, for example, 200C. and above so that agglomeration of the islands will occur. When the discontinuous thin film is cooled to room temperature, no further agglomeration will take place and a stable film will result. Film stability can also be achieved by deliberately placing nuclei on the surface of the ferroelectric body 11 prior to deposition of the noble metal. The nuclei may be in the form of small islands of refractory metal which have been oxidized, or even larger islands of a lesser refractory metal such as nickel which have been oxidized to form stable nuclei.

lf desired, a resistor 27 can be formed from the discontinuous film 26. This can be accomplished eitherby depositing the discontinuous thin film through a mask to provide the convoluted resistor 27 or, alternatively, the discontinuous thin film can be deposited over the entire surfaceand thereafter conventional photolithographic techniques can be utilized in conjunction with a mask to remove the undesired portions of the discontinuous film. After the discontinuous thin film has been formed into the desired pattern, a porous cover coat 28 can be deposited over the discontinuous thin film and over the exposed surfaces of the body 11. Such a porous cover coat can be formed by evaporating silicon monoxide in a high pressure of an inert gas such as argon to a thickness of several thousand Angstroms. This will provide mechanical protection for the discontinuous film of noble metal and still be porous enough to allow fast equilibrium between the discontinuous thin film and the environment in which the measurements are to be carried out, whether it'be gaseous or liquid, so that the actual gas or liquid can reach the surface of the ferroelectric body 11 and bring about a change in the electrical nature of the surface of the ferroelectric body. Holes 29 are etched into the porous cover 28 to expose the electrodes 17 and 18. Thereafter, a suitable metal such as aluminum or gold is evaporated onto the surface of thecoating '28. and the undesired metal removed by conventional photolithographic and etching techniques to provide metal leads 31 and 32 onthe coating 28 which extend downwardly through the holes 29 to make contact with the electrodes 17 and 18.

Operation of the completed ion-sensitive solid-state device may now be briefly described as follows. Let it be assumed that the device has been connected to an electrical circuit of the type shown in FIG. 4 in which the meter R gives an indication of the resistance of the discontinuous thin film resistor 27. A battery B has been provided which is connected to-a switch 36 that is adapted to be connected to terminals 1 and 2 of the switch to either the positive or negative side of the battery. Switch 36 in turn is connected to electrode 31. The mid-point of the battery B is connected to the electrode l9.

Let it be assumed that a gas is flowing over the porous covering 28 and is able to penetrate the cover 28 to make contact with the surface of the'ferroelectric body 11. The ferroelectric body can be polarized either up or down depending upon the polarity applied to the contact 31. As indicated by the arrow 37, the polarization will be characterized as being positive when the positive end of the dipoles in the ferroelectric body 1 1 are pointing at the discontinuous thin film and the polarization will be indicated as being negative as shown by the arrow 38 when the negative ends of the dipoles are pointing away from the discontinuous thin film.

It is well known to those skilled inthe art that a change in the surface electrical properties of the ferroelectric body will indeed change the resistivity of the discontinuous thin film resistor 27. When a voltage is applied across the ferroelectric body between the discontinuous thin film and the counterelectrode 19, the ferroelectric body will be permanently polarized either up or down when the electric field is of the order of l0 volts per cm. or greater. This polarization requires compensation at the surface of the ferroelectric body so that the dipoles will be charge compensated. This compensation can occur by adsorption of ions from a gas phase on the surface of the ferroelectric body 11. This gas phase will occur when gas is passing over the top surface of the porous layer 28 as shown in FIG. 3 and will penetrate the porous layer and will be present on the surface 12 of the ferroelectric body 11. The adsorbed ions will be negative ions because the positive ends of the dipoles are up and facing the discontinuous thin film resistor 27.

The adsorption of these negative ions will affect the transfer of electrons from island to island of the discontinuous thin film via a tunneling mechanism described in the literature and thus change the resistivity. This change in resistivity is indicated by the meter R.

When the polarization of the ferroelectric body is reversed from up to down by movement of the switch 36 from the negative to the positive terminals, the negative ions originally adsorbed will find themselves opposed by the negative endsof the dipoles which will tend to force them to desorb and to cause positive ions to be adsorbed to take their place. The change in resistance of the discontinuous thin film resistor 27 created by this change in ion population in the resistor 27 will be recorded by the meter R. Thus, by changing the polarization on the ferroelectric body 11, it is possible to obtain a change in the value of the resistance of the discontinuous thin film resistor 27. The magnitude of the change in the value of resistance and the rate of change in the value of resistance will be characteristic of the number of ions supplied by the gas stream passing over the discontinuous thin film resistor 27.

The actual rate of reversal for the switch 36 between up and down polarization will be affected by the rate at which equilibrium can be established between the gas phase and the surface of the ferroelectric body.

Although the example has been described in connection with a gas phase, it is readily apparent that, if desired, a liquid phase can be utilized in the same manner in which ions from the'liquid phase would pass through the porous covering at 28.

Reversals between one time a second and one time per minute should be sufficient to establish equilibrium between the gas or liquid stream and the ions adsorbed on the surface of the ferroelectric body 11.

The resistance of the discontinuous thin film resistor 27 would increase when the ferroelectric body is polarized up due to the adsorption of negative ions. The initial up polarization would result in a non-equilibrium surface which would attract negative ions. These ions, in turn, would oppose electron transfer resulting in an increase in resistance to a new equilibrium value higher than the initial value. A reversal in polarization, that is, to down polarization, would cause the negative ions to desorb and for the positive ions to adsorb. The positive ions would aid electron transfer resulting in a decrease in resistance and a new equilibrium resistance for the discontinuous thin film resistor 27 less than the original value of resistance would be registered by the meter R.

By way of example, let it be assumed that a gas to be analyzed is either an oxidizing or reducing gas. The oxidizing agents such as oxygen or oxides of nitrogen would tend to give morene'gative ions because oxygen inherently tends to form negative ions, whereas reducing agents such as hydrocarbons and the like tend to form positive ions. The resistance change of the discontinuous thin film would be an immediate indication of the oxidizing or reducing conditions to which the device has been exposed. v

It is possible that the porous layer 28 may be susceptible to change such that it will preferentially favor the adsorption of one-type of ion over another. In such case, the-device will be specific for that type of ion. In addition, the sensing device could be made sensitive to only one type of ion by ion selective membranes of the type known to those skilled in the art. FIG. 6 shows another embodiment of the invention which is particularly adaptable for use where the sensor will be located in a liquid flow. The encapsulant 41 would completely surround the ferroelectric body as well as the electrodes 17, 18 and 19, leaving only exposed the porous layer 28 overlying the discontinuous thin film 26. Suitable encapsulants would be a low melting point glass or an organic material such as an epoxy or a silicon rubber. Leads 42, 43 and 44 make contact with the electrodes.

Another embodiment of the invention is shown in FIG. 7 in which the ferroelectric body has its bottom surface 13 mounted directly upon a semiconductor device 51. The semiconductor device 51 can be of a conventional type. For example, it can consist of a semiconductor body 52 of a suitable type such as silicon into which have been diffused source and drain regions 53 and 54 extending downwardly from a planar surface 55 to form a channel 56 therebetween immediately below the surface. The ferroelectric body 1.1 is mounted over the channel 56 in an opening formed in a silicon dioxide insulating layer 57 formed on the surface 55 and overlaps at least a portion of each of the source and drain regions 53 and 54 as shown in FIG. 7. Contacts 58 and 59 extend through the layer 57 and make contact with the source and drain regions 53 and 54. The device has been connected to a battery B as indicated so that either a positive or negative polarity can be applied to the electrode 32 of the device 11.

In the embodiment of the invention in FIG. 7, in

order to make the device operate at much lower voltages, it is desirable that the body 11 be much thinner. For example, it should be reduced from h to 1 mil thickness to approximately 1 micron which would reduce the required voltage to 4 to 10 volts instead of to 200 volts as required in the previous embodiments. This would make the device suitable for direct integration into integrated circuits with compatible voltage requirements. Operation of the device shown in FIG. 7 may not be briefly described as follows. The ferroelectric body 11 can be polarized up or down in the manner hereinbefore described. Any ions adsorbed by the ferroelectric surface 12 and the discontinuous thin film will affect the charge compensation of the ferroelectric body which, in turn, will create a difference in the surface potential of the semiconductor body 52 underlying the body 11 and in the channel 56 so that the semiconductor surface conductance is a monitor of the surface potential created by the ions on the ferroelectric body 11. The ions which are adsorbed by the ferroelectric body induce a depletion or accumulation layer at the surface of the semiconductor body depending upon the carrier type which is in the majority in the semiconductor body and the source-drain conductance as read by the meter R in FIG. 7 will detect this accumulation or depletion. In other words, the discontinuous thin film device acts as the gate electrode of an M18 field effect transistor. After polarization of the ferroelectric body 11, the source-drain conductance can be monitored as a function of time to determine the amount and type of adsorbed gas.

From the foregoing, it can be appreciated that, if desired, one of the devices 11 can be placed on a semiconductor body to monitor the surface conductance of the semiconductor bodymerely by measuring the resistance between the source and the drain.

It is apparent from the foregoing that there has been provided a solid-state sensor or ion-resistive device which is relatively simple and which can be utilized in either gases or liquids for measuring type and concentration of ions. For example, it could be used for monitoring a catalytic muffler in an automobile to determine whether or not the catalytic muffler is working satisfactorily by analyzing the exhaust gases from the automobile. This device can determine whether or not the catalytic muffler was removing the hydrocarbons and the reducing agents from the exhaust of the automobile. The ion-sensitive solid-state device tends to average the oxidizing/reducing constituents in any gas mixture.

Hence, it is useful in monitoring the level of hydrocarbons in the exhaust gas of an internal combustion engine. The device is also useful in such an application because it can operate in high temperature environments.

Although the invention has been described utilizing principally noble metals for the formation of the discontinuous thin film, it should be possible to utilize oxidized metals as, for example, an oxidized metal in which the metal islands are protected by their own 7 oxide from further oxidation. Oxidized nickle or oxidized chromium should be satisfactory for this purpose.

The device is relatively simple. Charge compensation at the ferroelectric surface occurs through ion adsorption. As a result, the charged species at the ferroelectric-gas interface are determined in large part by the composition of the gas phase. Equilibrium between adsorbed species on the metal islands and the ferroelectric and also with the gas phase gives rise to a unique resistivity characteristic of the gaseous mixture present. Renewal of the sensor is accomplished by reversing the polarity across the ferroelectric body so that adsorbed ions encounter like charges on the dipoles facing them. The ions desorb because of electrostatic repulsion, leaving a surface which attracts ions of opposite charge. The resistance in the discontinuous thin film after equilibrium is reestablished providing additional information useful in determining the mixture of gases present.

I claim:

1. In an ion-sensitive solid-state device, a ferroelectric body having first and second surfaces, a discontinuous thin film on one of said surfaces, said discontinuous thin film being formed of islands generally having a size ranging from 100 to-5,000 Angstroms and an interisland spacing ranging generally from 10 to 100 Angstroms, electrodes disposed on said one side of the body and connected to the discontinuous thin film and an electrode on the other side of the body,'whereby a voltage can be applied to said electrodes to polarize the body in a direction toward or away from the discontinuous thin film so that the discontinuous thin film will alternatively adsorb either positive or negative ions.

2. A device as in claim 1 wherein said discontinuous thin film is formed of a noble metal.

3. A device as in claim 1 together with a porous covering over said discontinuous thin film, said porous covering being adapted to permit gases and liquids to pass therethrough to come in contact with said one surface of the body.

4. A device as in claim 1 wherein said discontinuous thin film is formed of an oxidized metal.

5. A device as in claim 1 together with means for encapsulating said body, said means for encapsulating said body having an opening overlying said discontinuous thin film.

6. A device as in claim 1 wherein said body has a thickness ranging from one-half mil to approximately one micron.

7 A device as in claim 1 wherein the body has a resistivity greater than 1X10 ohmscm. and remnant polarizability greater than one microcoulomb per square cm.

8. A device as in claim 1 wherein said body has a resistivity of 10 ohmcm. and a remnant polarizability of 50 microcoulombs per square cm. 

1. In an ion-sensitive solid-state device, a ferroelectric body having first and second surfaces, a discontinuous thin film on one of said surfaces, said discontinuous thin film being formed of islands generally having a size ranging from 100 to 5,000 Angstroms and an inter-island spacing ranging generally from 10 to 100 Angstroms, electrodes disposed on said one side of the body and connected to the discontinuous thin film and an electrode on the other side of the body, whereby a voltage can be applied to said electrodes to polarize the body in a direction toward or away from the discontinuous thin film so that the discontinuous thin film will alternatively adsorb either positive or negative ions.
 2. A device as in claim 1 wherein said discontinuous thin film is formed of a noble metal.
 3. A device as in claim 1 together with a porous covering over said discontinuous thin film, said porous covering being adapted to permit gases and liquids to pass therethrough to come in contact with said one surface of the body.
 4. A device as in claim 1 wherein said discontinuous thin film is formed of an oxidized metal.
 5. A device as in claim 1 together with means for encapsulating said body, said means for encapsulating said body having an opening overlying said discontinuous thin film.
 6. A device as in claim 1 wherein said body has a thickness ranging from one-half mil to approximately one micron.
 7. A device as in claim 1 wherein the body has a resistivity greater than 1 X 1012 ohmscm. and remnant polarizability greater than one microcoulomb per square cm.
 8. A device as in claim 1 wherein said body has a resistivity of 1020 ohmcm. and a remnant polarizability of 50 microcoulombs per square cm. 